Single frequency network random access

ABSTRACT

A method for controlling access to a radio channel in a single frequency network in which multiple base stations transmit the same data simultaneously to a user equipment, UE device, includes a first plurality of base stations transmitting a set of random access parameters of the single frequency network. The method also includes a second plurality of base stations receiving a random access preamble transmitted by the UE device. The second plurality of base stations the same or a subset of the first. The method also includes transmitting responses to the random access preamble from a third plurality of bases stations. The third plurality of base stations the same as or a subset of the second. The method also includes a fourth plurality of base stations receiving a scheduled transmission in response to the responses. The fourth plurality of base stations the same as or a subset of the third.

The present invention relates to a mechanism for performing randomaccess to a single frequency network, SFN.

In a single frequency network, several base stations transmit datasimultaneously by using same resources. These base stations behave as asingle frequency network and therefore appear as one single cell to themobile devices. In addition, base stations can be added to and removedfrom the set of base stations that are currently transmitting data to aspecific mobile device (user equipment, UE) according to UE's movementto cover the area where the UE is expected to move next, i.e. withregard to transmission to a specific UE some cells are switched on andsome are switched off according to UE's movement. If there is no needfor a base station to transmit to any UE it may also be totally switchedoff (powered down).

In the context of this invention the term SFN may be understood as a setof synchronously operating base stations that typically cover an area oflarger extension, but also as a sub set of base stations from thislarger set (so-called SFN Clusters). In order to avoid interferencebetween neighbouring SFN Clusters, resources being used in therespective SFN Clusters may be orthogonal to each other.

The present invention relates primarily to uplink traffic. It isconcerned with how to efficiently configure and establish initialconnection setup, i.e. the random access procedure for a singlefrequency network.

Random access procedures are known in the art and vary between radioaccess technologies. For example in LTE, described in 3GPP TS 36.321, amessage flow for random access comprises

-   -   (1) A UE reads the system information, which is broadcasted by        an eNB in each cell. Beside others the following parameters for        random access are received:    -   available PRACH resources (time slots) for the transmission of        the random        -   access preamble        -   available random access preambles        -   initial preamble power        -   size of random access response (time) window    -   (2) After the UE has decided to use the random access procedure,        it randomly selects a preamble and a resource from the available        preambles and resources    -   (3) The UE transmits a random access preamble    -   (4) The eNB that is chosen by the UE to serve it (i.e. the eNB        “on which the UE is camping”) receives the preamble. Only one        eNB can receive the random access preamble, as neighboured eNBs        are intentionally using different preambles.    -   (5) The eNB prepares and transmits a random access response. The        transmission timing is flexible but has to be done within the        configured random access response window.    -   (6) After reception of the response, the UE prepares and        transmits a scheduled transmission.    -   (7) After reception of the scheduled transmission, the eNB        prepares and transmits a contention resolution message.

CN 102196518 B describes a cell switching procedure including a randomaccess procedure. EP 2 534 873 A2 describes a further random accessprocedure in an LTE system in particular in connection with minimizationof drive test, MDT, measurements.

In US 2013/0089034 A1, a method is described for selecting one out of aplurality of base station to serve a UE in an uplink, UL. A single basestation already serving the UE in a downlink, DL, controls the method,which involves by the UE sending reference signals in the UL to multiplebase stations. The base stations receive and decrypt the signals andsend a received signal strength to the controlling base station forselection of the one base station to serve the UE in UL. The selectionis fixed; the selected base station serves the UE.

In a known random access procedure, for example in LTE, the UE has toselect a base station before a random access request can be sent.Therefore the UE has to regularly perform cell selection in idle mode tofind and select the best-suited base station. These idle mode proceduresare power consuming for the mobile device. Furthermore, the receivingquality of a single base station is worse compared to the inventivemultiple base station concept (i.e. as offered by an SFN Cluster). Theaccess procedure according to the prior art results in unsuccessful andpower wasting transmissions being more likely.

Where multiple base stations build an SFN-like synchronous sub-net, theknown conventional random access concepts will fail, as base stationsare intentionally disabled to receive random access preambles fromneighboured base stations.

WO 2014/204365 A1 describes a method for controlling multiple antennapoints by a network node. The network nodes form picocells andaccordingly use orthogonal radio resources and do not form a singlefrequency network in which multiple access points transmit the samesignal using the same radio resources.

WO 2013/178612 A1 describes timing advance management in the presence ofrepeaters and remote radio heads, the remote radio heads being served byan eNB. A UE received signals from multiple radio heads and radio headsfor which communications have similar timing advances are assigned to atiming advance group for timing advance management. There is noindication that the radio heads form a single frequency network.

US 2013/0170385 A1 describes a method for contention resolution in amobile communication system in which a UE receives BCH signals broadcastby two base stations using a multimedia broadcast single frequencynetwork radio transmission format.

The present invention provides a method for controlling access to aradio channel in a single frequency network in which multiple basestations transmit the same data simultaneously to a UE device accordingto claim 1.

The invention also provides a method for a user equipment, UE, device toaccess a radio channel in a single frequency network in which multiplebase stations transmit the same data simultaneously according to claim2.

Further preferred aspects of the methods of the invention are providedaccording to the dependent claims.

In a further aspect, the invention provides a UE device, adapted toaccess a radio channel in a single frequency network in which multiplebase stations transmit the same data simultaneously according to claim10.

In a still further aspect, the invention provides a single frequencynetwork radio access, SFN-RA, controller arranged to control a pluralityof base stations forming the single frequency network in order that theplurality of base stations each transmit simultaneously according toclaim 12.

The invention provides a random access procedure where multiple basestations (eNB) are enabled to receive and respond to a random accesspreamble. This is advantageous, as due to the multiplereceiver/transmitter concept, the likelihood of successful transmissionis increased. Therefore, access is faster, battery consumption of themobile device is reduced and radio resources (for re-transmissions) aresaved. In addition the SFN random access concept is advantageous, as theexecution of mobility related idle mode procedures can be reduced, i.e.the UE has to read the (random access portion of the) System Informationvery rarely, as the initial random access configuration remains valid(even when the UE moves) as long as it stays in the respective SFNCluster.

The invention provides the following benefits.

The SFN-RA controller (a functional entity that may for example be partof the SFN Cluster Management Unit or the Resource Control Unit) isenabled to configure random access relevant parameters commonly for allsmall cells within a single frequency network (or an SFN Cluster). Allbase stations within an SFN (or SFN Cluster) are enabled tosimultaneously receive a random access preamble. These leading to a morereliable reception of the random access preamble

All base stations within an SFN (or SFN Cluster) are enabled tosimultaneously respond to a received random access preamble such thatthe reception of the random access response is more reliable.

All base stations within an SFN (or SFN Cluster) are enabled to forwarda received “scheduled transmission” to an SFN-RA controller. The SFN-RAcontroller is enabled to combine multiple received “scheduledtransmissions” and to resolve any conflicting information therein. Theseprovide the advantage of a more reliable reception of the “scheduledtransmission”.

The SFN-RA controller is enabled to instruct all or a subset of the basestations of the SFN (or SFN Cluster) to simultaneously transmit the“contention resolution” message. The decision which base stations are totransmit this message is made based on reception quality reported by thebase stations, e.g. a subset of base stations may be sufficient for ahighly reliable collective DL transmission of a message. The basestations are enabled to receive the contention resolution message fromthe SFN-RA controller and to forward it to the UE by using the resources(time slot and subcarrier) as instructed by the SFN-RA controller. Theseprovide the advantage of a more reliable reception of the “contentionresolution” message

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 shows a schematic diagram of a single frequency network;

FIG. 2 shows a message exchange between an SFN controller, small cellsand a UE; and

FIG. 3 shows a message exchange including and after a random accesspreamble transmission.

For illustrating the invention, FIG. 1 shows an exemplary schematicrepresentation of a single frequency network arrangement 10 comprisingtwo clusters, a cluster M and a cluster N. Three small cells SC_n toSC_n+2 are shown configured for the mobile device “UE1” as SingleFrequency Network (SFN Cluster N). Two small cells SC_m and SC_m+1 areshown configured to form the second cluster, SFN Cluster M. Each of thesmall cells SC_n to SC_n+2 and SC_m to SC_m+1 are connected to a SFNrandom access SFN-RA controller 12 which configures the small cells. Asshown in FIG. 1, the SFN-RA controller 12 is part of a cluster specificradio control unit, RCU, 14 which is in turn connected to a singlefrequency network cluster management unit 20.

As illustrated, the SFN cluster management unit 20 comprises a SFNController 22, a position determination unit 24, a central RCU 26 and acluster partitioning unit 28. The SFN cluster management unit 20 isfurther in connection with an LTE mobility management entity, MME, 30,here via a 51 interface 32. Rather than having individual SFN-RAcontrollers attached to each cluster specific RCU, a central SFN-RAcontroller can be associated with the central RCU 26.

It should be noted that the term “small cell” is used which may, forexample be a Node B, NB, evolved Node B, eNB or other forms of basestation.

It is a first aspect of the SFN random access procedure, that all basestations of an SFN (or SFN Cluster) use the same random accessconfiguration. Eight parameters (some of which are already used in theknown LTE random access procedure) are configured for the base stationsby the SFN-RA controller. In contrast to the known procedure, the basestations implementing the SFN random access procedure do not have thefreedom to select the parameters on their own

-   -   (1) available random access resources (time slots) for the        transmission of the random access preamble    -   (2) available set of random access preambles    -   (3) initial preamble transmit power    -   (4) size of random access response window    -   (5) backoff parameter value

In addition, the following new parameters are configured by the SFN-RAcontroller:

-   -   (6) requirements to transmit (positive) random access response    -   (7) timing to transmit the random access response message    -   (8) timing alignment configuration. The timing alignment is the        timing to be used by the UE to transmit signals earlier in order        to result in a synchronized reception of signals from different        UEs by the base station.

The SFN-RA controller configures the parameters as follows:

Parameters (1) and (2) are selected based on the current capacity needfor random access. More time slots and preambles can be made availableif more capacity is required. Therefore re-configurations are done ifthe capacities need changes.

Parameter (3) is selected, so that in most cases the first transmittedpreamble can be correctly received. The nominal value of this parameterdepends on the size of the cell's coverage area. The concept of multiplereception points is to be taken into account here as our new concept mayincrease the perceived coverage area significantly.

Parameter (4) is selected based on the capability of the configured setof base stations to respond to all random access requests within thistime window.

Parameter (5) indicates a backoff parameter value as used in the randomaccess backoff procedure. With this parameter the base stations may beinstructed by the SFN-RA controller to initiate a backoff algorithm inthe requesting UEs to delay the next random access preamble transmissionattempt(s). The random access process stops when the maximum number ofrandom access preambles has been transmitted in uplink direction withoutany positive feedback from the base stations.

Parameter (6) indicates in which cases the configured set of basestations sends a positive response to the requesting UE, i.e. a randomaccess response message that will allow the UE to proceed with the“scheduled transmission” message in uplink direction. For example if thepreamble is received correctly, if no contention is detected, and ifresources on the air interface and on the interface to the core networkare free, this will be a valid case to send a positive response.

Parameter (7) indicates which time slot has to be used for transmissionof the response message.

Parameter (8) indicates the method to derive the timing alignment value(as described below). In case method 1 is configured, the timingalignment value to be used is included. This is a fixed value used byall base stations.

The SFN-RA controller then transmits (at least one of) the parameters toeach base station as a configuration message. All base stations of thesame SFN (or SFN Cluster) will obtain the same set of parameters. Basestations from another SFN (or SFN Cluster) will obtain parameters thatcould be identical or different from the parameters of other SFNs (orSFN Clusters).

After the base stations of SFN Cluster N have received the configurationmessage from the SFN-RA controller containing some, all or more than theeight parameters as described above, they are implemented. The basestations configure their receiver for the reception of random accesspreambles and the transmitter for broadcasting the random accessparameters, which are relevant for the UEs. Having done so, they areready to receive any of the available random access preambles at theconfigured time slots.

In the example arrangement of FIG. 1, the small cells SC_n to SC_n+2will start to broadcast the random access parameters synchronously aspart of a system information broadcast, SIB, i.e. by using same timeslots and same subcarriers.

Referring to FIG. 2, a typical exchange of messages between the SFN-RAcontroller and the small cells as well as between the small cells and aUE is illustrated. The configuration message from the SFN-RA controllerto each of the small cells is shown as messages 34′, 34″ and 34′″. Inturn, the small cells broadcast the SIBs illustrated by messages 36′,36″ and 36″. Message 36″ is shown as a dotted line to indicate that theparameter is transmitted but will not contribute significantly to areceived signal at the UE1 due to a large distance between the smallcell and the UE.

Note, that more parameters are required by the UEs for random access asdescribed in 3GPP TS 36.321, which are also broadcast. For the sake ofsimplicity they are not further described here as they are used as usualfor normal random access.

In the LTE random access procedure, the timing alignment value isderived dynamically by a base station based on a reception timing of arandom access preamble. This method is not applicable in a SFN, as ittypically leads to different timing alignment values for each basestation. Synchronous transmission is not possible when the data (i.e.the timing alignment value) are different for each base station. Thisissue is solved by one of the three following methods:

Semi Static Value:

The SFN-RA controller configures the timing alignment value. It selectsthe value depending on the average distance from any UE in the SFN tothe nearest base station, e.g. r/2 where r is the radius of the coveragearea. This value is transmitted to the base stations and used in therandom access response message. This method is advantageous, as itenables a fast response by the base stations and reduces the amount ofsignalling.

Dynamic Value with Coordination by SFN-RA Controller:

After reception of the random access preamble by the base station, thetime offset delta_T between the received preamble and the downlinktiming is calculated. This value is transmitted to the SFN-RAcontroller. The SFN-RA controller selects one value for the timingalignment based on the multiple received time offsets. For example thetiming alignment value is derived by only considering the lowest timeoffset value. Alternatively, a mean or average offset value could beused to determine the alignment value.

Dynamic Value with Selection by UE:

After reception of the random access preamble by the base station, thetime offset delta_T between the received preamble and downlink timing iscalculated and used to derive a timing alignment value. Each basestation will derive an own value. This value is transmitted to the UEwithin the random access response message. The transmission is donesimultaneously by all base stations. Without further means, thereception of these multiple different messages on same resources willfail. Therefore, orthogonal codes are used, to enable that the differentvalues could be distinguished at the UE. E.g. different orthogonalspreading codes are assigned by the SFN-RA controller to the basestations, which will spread the timing alignment value by using theassigned code. These codes are known in each UE, e.g. they arepreconfigured. After reception of the response message by the UE, itwill decode the different timing alignment values and will calculate onevalue to be used for the following transmission. E.g. it uses the lowestvalue or a mean or average value.

The random access procedure performed by the UE will now be describedwith reference to FIG. 3.

As a prerequisite it is assumed that a UE, UE1, has received the UErelevant parameters required for the random access from any (ormultiple) base station(s) in the SFN Cluster.

(1) UE1 selects a random access preamble and a time slot from theconfigured set and transmits the preamble with the configured power. Asshown, SC_n and SC_n+1 receive the preamble correctly. SC_n+2 does notreceive the preamble (indicated by the doted line) because, for examplea large UE-base station separation. SC_n and SC_n+1 decide to send apositive acknowledgement response, as the preconditions of theconfigured parameter 5 are fulfilled. SC_n and SC_n+1 generateparameters for a response message as instructed by the SFN-RAcontroller.

(2) SC_n and SC_n+1 send the random access response messagesynchronously to UE1. They use exactly the time slot for transmission ofthe response message which has been configured by the SFN-RA controller.

Note: In this step both base stations (e.g., eNBs) have individuallygenerated the same response for this request and they all use the sameresources for transmission. Unlike in known random access responsetransmission the base stations do not have any freedom to choose atransmission timing, the transmission window is just a parameterindicated by the SFN-RA controller to schedule the transmission by thebase stations and used by the UEs to stop detecting a response message.This ensures the SFN-like transmission and a timely response to therandom access attempt of UE1 that would not be possible when the basestations would coordinate their joint response before transmission. UE1receives the response message without identifying individualtransmission points (base stations SC_n and SC_n+1).

(3) UE1 transmits the scheduled transmission message, which is assumedto be received by SC_n and SC_n+1 in this example.

(4) SC_n and SC_n+1 forward the message to the SFN-RA controllerincluding information about the reception quality (e.g. UL signalstrength). The SFN-RA controller combines the possibly multiple receivedmessages while considering the reception quality to prepare a common“contention resolution” message.

(5) The SFN-RA controller selects a set of suitable base stations thatshould transmit the common “contention resolution” message to UE1. Thisselection could be based on the reception quality, i.e. only the smallcell(s) with highest reception quality is/are selected to transmit the“contention resolution” message. The SFN-RA controller transmits the“contention resolution” message to the selected small cells. In theexample of FIG. 3 only SC_n and SC_n+1 were selected. When the“contention resolution” message is exchanged between the SFN-RAcontroller and the selected base stations, information may be includedabout the resources to be used by the base stations for transmission ofthe “contention resolution” message to UE1

(6) SC_n and SC_n+1 transmit the contention resolution messagesynchronously as instructed by the SFN-RA controller. After successfulreception by UE1 the random access procedure is complete.

The multiple reception point concept of the above random accessprocedure leads to new situations, which have to be handled by themobile network.

If the scheduled transmission is not correctly received by one or morebase stations, the SFN-RA controller will resolve this issue. The basestations forward the received messages and an indicator of the receptionquality (or the perceived UL signal strength, or a reliabilityindication, etc.) to the SFN-RA controller as described above. TheSFN-RA controller will then discard messages with low reception qualityand will only use messages with good reception quality. In anotherembodiment, the base station will forward the message with so-called“soft bits”. That means, that the base station does not decode thereceived message to binary bits (“0” or “1”). Instead it forwards onlythe received symbols to the SFN-RA controller where the actual decodingtakes place. The SFN-RA controller will combine the “soft bits” from allbase stations while considering the reception quality (or the perceivedUL signal strength, or a reliability indication, etc.) of each instanceof the received message and will than decode the message. This will leadto the best receiver performance.

Preamble collision may also occur due to transmissions from multipleUEs. The current behaviour of the known random access procedure is thefollowing: In case that two or more UEs are transmitting the samepreamble simultaneously (i.e. using the same time slot) to the same basestation, all but one requests will be rejected by the base station bytransmission of a corresponding “contention resolution” message. Therejected UEs would have to initiate the random access procedure one moretime.

The present random access procedure behaves differently. In case thattwo or more UEs transmit the same preamble simultaneously in the sameSFN to different base stations, the base stations will respond withindividual messages, so that the UEs will proceed with their individual“scheduled transmission” in the uplink direction. These messages are allforwarded to the SFN-RA controller. The SFN Controller detects thatthese messages are originating from different UEs (based on the includedUE IDs). It will in this case not combine the multiple messages to asingle message but will interpret them independently and will assigndifferent resources for each UE to the base stations for the submissionof the “contention resolution” message in DL direction. Therefore theinventive method is advantageous as it will lead to lower number ofrejected requests and will therefore save radio resources and batterypower.

1. A user equipment, UE, device, adapted to access a radio channel in asingle frequency network in which multiple base stations transmit thesame data simultaneously, wherein the UE device is configured to:receive from a first plurality of base stations a set of random accessparameters common to the plurality of base stations of the singlefrequency network; transmit a random access preamble; receive aplurality of responses to the random access preamble the plurality ofresponses having been transmitted simultaneously from a second pluralityof base stations of the single frequency network, wherein the secondplurality of base stations are the same as or a subset of the firstplurality of base stations and the plurality of responses transmitted bythe second plurality of base stations are the same; and transmit ascheduled transmission in response to the plurality of responses,wherein the UE device is arranged to determine an actual timingalignment value by receiving multiple timing alignment values from thesecond plurality of base stations of the single frequency network inresponse to the random access preamble and to determine the actualtiming alignment value from the multiple received timing alignmentvalues.
 2. The UE device of claim 1, wherein the UE device is configuredto determine the actual timing alignment value by selecting one of alowest value, a mean value and an average value of the multiple receivedtiming alignment values.
 3. A user equipment, UE, device, adapted toaccess a radio channel in a single frequency network in which multiplebase stations transmit the same data simultaneously, wherein the UEdevice is configured to: receive from a first plurality of base stationsa set of random access parameters common to the plurality of basestations of the single frequency network; transmit a random accesspreamble; receive a plurality of responses to the random access preamblefrom a second plurality of bases stations of the single frequencynetwork, the second plurality of base stations being the same as or asubset of the first plurality of base stations; and transmit a scheduledtransmission in response to the plurality of responses.
 4. The UE deviceaccording to claim 3, wherein the UE device is configured to determinean actual timing alignment value by receiving timing alignment valuesfrom the second plurality of base stations of the single frequencynetwork in response to the random access preamble and to determine theactual timing alignment value from the received timing alignment values.